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SUMMARY
Evaluation of Safety Strategies
at Signalized Intersections
Background
In 1997, the American Association of State and Highway Transportation Officials (AASHTO)
Standing Committee for Highway Safety along with the Federal Highway Administration
(FHWA), National Highway Traffic Safety Administration (NHTSA), and the Transportation
Research Board (TRB) Committee on Transportation Safety Management convened a meet-
ing of national experts in the highway safety area to develop a Strategic Highway Safety Plan.
This plan focuses on 22 highway safety challenges or emphasis areas that have an impact on
highway safety. To advance the implementation of countermeasures to reduce accidents and
injuries, National Cooperative Highway Research Program (NCHRP) Project 17-18(3)
began the development of a series of implementation guides which were subsequently
published in the form of several volumes of NCHRP Report 500: Guidance for Implementation
of the AASHTO Strategic Highway Safety Plan. Each guide addresses one of the 22 emphasis
areas and includes an introduction to the problem, a list of objectives for improving safety
in that emphasis area, and strategies for each objective. Each strategy is designated as proven,
tried, or experimental.
Objectives
Many of the strategies in NCHRP Report 500 have not been evaluated using state of the
art methods. The objective of this project was to evaluate the safety effectiveness for selected
strategies identified in NCHRP Report 500: Guidance for the Implementation of the AASHTO
Strategic Highway Safety Plan, Volume 12: A Guide for Reducing Collisions at Signalized
Intersections (Antonucci et al., 2004). The intent is to develop reliable Crash Modification
Factors (CMFs).
Approach
The first part of the project was a critical review of published studies for each treatment/
strategy. To obtain information about ongoing or planned research, several research-in-progress
databases were reviewed in addition to discussions with other highway safety researchers,
and conversations with research sponsors such as FHWA and the Insurance Institute for
Highway Safety (IIHS).
Following the literature review, surveys were developed and sent to two listservs, contacts
in all 50 States, and several local agencies. The respondents were asked to indicate if they
implemented a particular strategy/treatment and the approximate number of installations
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for each strategy/treatment. In addition, they were also asked to rate the importance of
knowing the Crash Modification Factor for each treatment. These ratings along with the
information about the installations were used to develop a short list of treatments for further
consideration. To get further information about the availability and suitability of the data
for the short-listed strategies, selected agencies who responded to the original web-based
survey were contacted by telephone.
Using the results of the literature and the survey of practitioners, a recommended and
prioritized list of strategies for evaluation were developed along with a work plan for evalu-
ating these strategies. The prioritized list of strategies and the work plan were discussed at
the Interim Meeting with the NCHRP Panel.
At the meeting, the project team and the NCHRP Panel agreed on a set of Tier I treatments
(higher priority) and a set of Tier II treatments (lower priority) for the evaluation. At the
meeting, it was agreed that funds will probably not be available to address all the treatments
in Tier I. In addition, if data were not available to evaluate one or more of the treatments
in Tier I, treatments in Tier II could be included. The following is a list of treatments in
Tier I and Tier II:
Tier I (higher priority)
· Protected phasing,
· Protected-permissive phasing,
· Modify phasing and add left-turn lane,
· Add left turn only,
· Lengthen left-turn lane,
· Dynamic advance warning flashers,
· Optimize clearance intervals, and
· Flashing yellow arrow.
Tier II (lower priority)
· Convert signalized intersections to roundabouts,
· Right-turn channelization,
· Add signal heads,
· Improving friction at approaches to intersections,
· Changing fonts, and
· Split phasing.
After the interim meeting, the project team started compiling the necessary data for
evaluating the treatments in Tier I. The project team could not find sufficient data for some
of the treatments in Tier I, and after discussing this issue with the panel, it was decided to
include one of the treatments from Tier II (convert signalized intersections to roundabouts)
for which there appeared to be a research need coupled with the availability of data for a
substantial number of treatment sites. The following is the final list of treatments that were
evaluated in this study:
· Install Dynamic Advanced Warning Flashers,
· Convert Signalized Intersections to Roundabouts,
· Increase Clearance Intervals,
· Change Left-Turn Phasing, and
· Introduce a Flashing Yellow Arrow.
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Results
The intent was to use the state of the art empirical Bayes (EB) method for evaluating the
safety impacts of these treatments. In addition, cross-sectional regression models were used
to derive CMFs if the sample size for the EB evaluation was limited, and to examine the
comparability of before-after and cross-sectional studies, a subject of topical interest in CMF
development, for which there is little research.
To evaluate the safety of dynamic signal warning flashers (DSWF), data were compiled from
North Carolina, Virginia, and Nevada. In Virginia and Nevada, flashers had been introduced
when the intersections were signalized, and hence did not allow the application of a before-
after method. The Virginia and Nevada data were used to develop cross-sectional regression
models, and these models were used to develop the CMFs. In North Carolina, a before-after
analysis was possible, but the results were not found to be reliable. Hence, the CMFs based on
the results from the cross-sectional models from Virginia and Nevada data are recommended
and provided in this report. These results show a consistent reduction in total crashes at
intersections that had DSWF. The results also suggest that DSWF may help to reduce angle,
injury, and heavy vehicle crashes. The expected safety benefits are statistically significant at
the 0.05 level for all crash types except heavy vehicles.
To evaluate the safety of converting from signalized intersections to roundabouts, the
before-after EB method was used to evaluate treatments in Colorado, Florida, Indiana,
Maryland, Michigan, North Carolina, New York, South Carolina, Vermont, and Washington.
A disaggregate analysis was conducted to identify differential effects based on specific site
characteristics (e.g., traffic volume, area type, and number of approaches). There was a sub-
stantial reduction in injury and fatal crashes following the implementation of roundabouts.
In terms of the effect on total crashes, the safety benefit of roundabouts appears to decrease
as traffic volumes increase. The analysis also suggested that the safety benefit is larger for
suburban than for urban conversions and for intersections with four approaches compared
to those with three.
For evaluating the change in clearance interval, data were obtained in California from the
cities of San Diego and San Francisco and in Maryland from the counties of Howard and
Montgomery. The primary analysis methodology used was the EB before-after analysis as
previously described. The evaluation analyzed the effects of the treatment on crash frequencies
for different crash types and severities before and after the treatment. Specifically, the EB
analysis was employed to investigate five specific scenarios. The following three scenarios
were related to various combinations of increasing the yellow and all red time:
1. Increasing both the yellow and all red phases.
2. Increasing the all red phase only.
3. Increasing the yellow phase only.
Two other scenarios were investigated, comparing the total change interval to the ITE
recommended practice. In both cases, the before condition was represented by signalized
intersections where the total change interval was less than the ITE recommended practice.
The after period was represented by signalized intersections with the following characteristics:
1. Total change interval remains less than the ITE recommended practice.
2. Total change interval is greater than the ITE recommended practice.
The analyses attempted to develop CMFs by severity (i.e., fatal/injury vs. total crashes)
and by crash type (i.e., total, angle, and rear-end) for both States. The EB before-after analyses
indicated a significant reduction in total, injury, and rear-end crashes under various scenarios.
Specifically, the analysis indicated a statistically significant reduction (at the 0.05 level) in
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total crashes as a result of (1) increasing the all red phase only and (2) increasing the total
change interval to be less than the ITE recommended practice. Injury crashes were significantly
reduced as a result of increasing the total change interval to be less than the ITE recommended
practice. Rear-end crashes were significantly reduced as a result of increasing the total change
interval to be greater than the ITE recommended practice. The change in angle crashes was
statistically insignificant under all scenarios investigated.
For evaluating the change from permissive to protected-permissive phasing, data from
59 intersections in Toronto and 12 intersections from urban areas in North Carolina were
used. The analysis methodology was the EB before-after method. Similar results were obtained
for the two jurisdictions. At both intersection and approach levels the results indicate sub-
stantial and highly significant benefits for the target crash type, involving a left-turn vehicle
and a through vehicle from the opposing approach. As expected, the benefit at the intersection
level is greater at intersections where more than one approach is treated.
One of the fundamental questions the study was expected to answer was the extent to
which the decrease in target crashes may be offset by a compensating increase in a non-target
crash type such as rear-end. At both the intersection and approach levels, there were small
percentage increases in rear-end crashes, which was statistically significant at the 0.05 level,
when the results of the two jurisdictions are combined.
To evaluate the safety of implementing flashing yellow arrow for permissive left turns, data
from urban areas in Oregon, Washington, and North Carolina were used. For Oregon and
Washington, data on reference sites were limited in most of the jurisdictions and hence the EB
methodology could not be applied with the required rigor: the methodology applied combined
some aspects of the empirical Bayes and Comparison Group approaches. For North Carolina,
data on reference sites were available and hence the EB methodology could be applied.
For Oregon and Washington, CMFs could be developed for total intersection crashes,
intersection left-turn crashes, and left-turn crashes on the treated approaches. For North
Carolina, CMFs could be developed for total intersection crashes, intersection injury and
fatal crashes, intersection left-turn crashes, and intersection left-turn opposing through
crashes. For the combined data from Oregon, Washington, and North Carolina, CMFs
could be developed for total intersection crashes and intersection left-turn crashes for the
following three scenarios based on the phasing in the converted legs in the before period:
· Permissive or combination of permissive and protected-permissive (i.e., at least 1 converted
leg was permissive in the before period) to flashing yellow Arrow (FYA) protected-
permissive (9 sites).
· Protected-permissive (all converted legs had protected-permissive in the before period)
to FYA protected-permissive (13 sites).
· Protected (i.e., all converted legs had protected in the before period) to FYA protected-
permissive (29 sites).
It was clear from the results that converting from protected phasing to FYA operation
(third scenario) leads to a dramatic increase in left-turn crashes. However, there appears to
be a benefit for both left-turn and total intersection crashes if there was permissive left-turn
operation in at least one of the legs in the before period (i.e., first scenario). The sites where
the converted legs were protected-permissive (second scenario) seem to have experienced a
smaller reduction in left-turn and total intersection crashes (compared to the sites in the first
scenario), but this reduction was not statistically significant at the 0.05 level. It is important
to note that the number of sites in each of the first two scenarios was limited and hence those
individual results should be treated with due caution.
